In the gel polymerization of absorbent polymers, i.e., water-swellable polymers, monomers are polymerized in aqueous solution. Certain additives, such as crosslinking agents, may be incorporated into the monomer mixture. The product of the polymerization process is typically dried and subjected to mechanical means of particle size reduction and classification including chopping, grinding, and sieving. During such particle size reduction, ultrafine particles or dust, i.e., particles having a mean diameter less than about 10 microns, are unavoidably created.
WO 94/22940, published Oct. 13, 1994, discloses a water-swellable polymer composition and process for the production thereof. The composition has reduced dusting tendencies and is attrition-resistant. The reduced dusting is due to coating the polymer particles with a dedusting agent selected from such hydrophilic compounds such as aliphatic polylols and polyalkylene glycols. WO 94/22940 states that employing compounds such as these does not detrimentally affect the performance of the polymer. However, it has been discovered that when the water-swellable polymer products of the process of WO 94/22940 are swollen with an aqueous fluid, the surface tension of the fluid in contact with the swellable polymer product and air is decreased from that of a fluid in contact with untreated polymer products. This is unfortunate because surface tension of the contacting fluid at equilibrium is one of the measures of the performance of a water-swellable polymer.
Surface tension of the contacting fluid at equilibrium is used as a measure of performance because water-swellable polymers yielding a fluid with higher surface tension generally tend to "wick" better, i.e. the polymer transports aqueous fluids by capillary flow more efficiently and holds the fluids more strongly. As U.S. Pat. No. 5,352,711 discusses, wicking is very important for absorbent devices such as diapers or sanitary articles. The effect of surface tension on capillary flow and capillary pressure is described more fully by the Lucas-Washburn equation as discussed by P. K. Chatterjee, "Absorbency", Elsevier, Amsterdam, 1985, pp. 36-37.
Industry would find great advantage in a water-swellable, attrition-resistant polymer composition which does not dust, e.g., which comprises no more than 2.5 parts per million (ppm) unassociated particles less than 10 microns in diameter, and which substantially maintains or increases the surface tension of aqueous fluids when the latter are in equilibrium with the composition and air as compared to compositions comprising greater than 2.5 ppm unassociated particles less than 10 microns in diameter. Industry would further find advantage in a new process which reduces the level of particles less than 10 microns in diameter in a given composition by at least 80 percent and/or provides a composition comprising no more than 2.5 ppm unassociated particles less than 10 microns in diameter while substantially maintaining or increasing the surface tension of an aqueous fluid when it is in equilibrium with the composition and air.
Accordingly, the subject invention provides a composition comprising gel-polymerized, water-swellable polymer particles, the particles bearing on their surface a physical coating of a hydrophobic dedusting agent, wherein the composition comprises no more than 2.5 ppm unassociated dust having a maximum diameter less than or equal to 10 microns. The surface tension of an aqueous fluid in equilibrium with the composition and air is substantially maintained or increased as compared to untreated gel-polymerized, water-swellable polymer particles. The composition absorbs at least 20 grams of a 0.9 weight percent aqueous saline solution under a pressure of 0.3 psi (21,000 dynes/cm.sup.2), i.e., a 60 minute 0.3 psi (21,000 dynes/cm.sup.2) AUL greater than 20 grams/gram as measured in accordance with the Absorption Under Load Test set forth in U.S. Pat. No. 5,147,343, incorporated herein by reference. The composition is attrition-resistant in that it contains less than about 100, preferably less than about 65 ppm of unassociated dust having a maximum diameter less than or equal to 10 microns after having been roll milled for 20 minutes.
The subject invention further provides a process which comprises:
(a) preparing a water-swellable hydrogel by gel polymerization; PA1 (b) drying and sizing the hydrogel to form a composition comprising dried and sized particles, the composition having a first amount of unassociated dust having a maximum diameter less than or equal to 10 microns; and PA1 (c) contacting the composition with an effective amount of a hydrophobic dedusting agent to form an attrition-resistant, dedusted composition, wherein the attrition-resistant, dedusted composition is characterized by: PA1 (a) placing the composition into the sample holder of a pulsed jet disperser; PA1 (b) blasting the composition with air dried by passage through dehydration means; and PA1 (c) determining the weight of particles in each of a plurality of size increments.
(1) a 60 minute 0.3 psi (21,000 dynes/cm.sup.2) AUL greater than 20 grams/gram whereupon the attrition-resistant, dedusted composition has a second amount of unassociated dust having a maximum diameter less than or equal to 10 microns, the second amount being less than about 20, preferably less than about 10, more preferably less than about 5, most preferably less than about 1 percent of the first amount and/or the second amount of unassociated dust being less than 2.5 ppm of the attrition-resistant, dedusted composition; PA2 (2) a tendency to generate a third amount of unassociated dust having a maximum diameter less than or equal to 10 microns upon roll milling of the attrition resistant, dedusted composition for 20 minutes, the third amount being less than or equal to the first amount, preferably less than about 100, more preferably less than about 65 ppm of the attrition resistant, dedusted composition; and PA2 (3) a surface tension such that when an aqueous fluid is in equilibrium with the attrition resistant, dedusted composition and air, the surface tension of the aqueous fluid is substantially maintained or increased as compared to an untreated gel-polymerized, water-swellable polymer composition.